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(1) Field of the Invention
The present invention relates to a process for producing alumina matrix carbide and boride reinforced ceramic composites wherein for any particular composite, the relative density is 97% or more of the theoretical density. The composites are prepared in a container wherein the interior surfaces of the container are graphite and have a protective coating consisting of a first layer comprising silicon carbide and boron carbide with a binder and a second layer comprising silicon carbide particles, wherein the protective coating prevents carbon bleed-through and provides a boride-containing equilibrium atmosphere during the process. The present invention further relates to an alumina-based ceramic composite which comprises a metal carbide preferably selected from the group consisting of silicon carbide, titanium carbide and zirconium carbide, and mixtures thereof, and a boride preferably selected from the group consisting of boron carbide, titanium boride, or zirconium boride, and mixtures thereof. Finally, the present invention relates to a protective coating for an article comprising a first layer of a silicon carbide and a boron carbide in a binder and a second layer comprising a silicon carbide particles wherein the protective coating is able to withstand repeated exposure to high temperature.
(2) Description of Related Art
Aluminum oxide (alumina) based ceramics which contain at least 50% alumina and carbides produced by hot pressing or hot isostatic pressing have high strength and excellent resistance to corrosion, oxidation, and wear. However, these alumina-based ceramics have poor strength and toughness when compared to silicon nitride-based ceramic materials. Of particular importance is that alumina-based ceramics generally have relatively poor strength and toughness and are sensitive to thermal crack formation because the aluminum oxide has relatively poor thermal conductivity. In the case of metal cutting tools, this leads to very short tool lives in machining steel, particularly under conditions with short operating times and varying cutting depths. Various attempts have been made to improve the strength, toughness, and thermal conductivity of alumina-based ceramics. To some extent, the thermal properties of alumina-based ceramics has been improved by addition of titanium carbide or titanium nitride to improve the thermal conductivity of the ceramic. The carbide/nitride also had the effect of increasing the hardness of the ceramic. However, the toughness of the ceramic material was insufficient for fabricating tools to use for cutting steel. When zirconium oxide was added to the aluminum oxide a composite was produced which had increased strength and toughness but with thermal properties not much better than those of pure aluminum oxide. The addition of silicon carbide to aluminum oxide has resulted in an alumina-based ceramic containing silicon carbide whiskers which has increased strength and toughness when compared to pure alumina-based ceramics.
Alumina-based ceramics have been described in the prior art. For example, U.S. Pat. No. 4,732,878 discloses an oxidation resistant alumina-based ceramic comprising alumina-silica or alumina-boria-silica as a first phase and an in situ generated discontinuous carbon second phase. U.S. Pat. No. 5,418,197 discloses fabrication of an alumina-based ceramic containing homogeneously dispersed silicon carbide whiskers. U.S. Pat. No. 5,538,926 discloses fabrication of alumina-based ceramic containing silicon carbide whiskers and one or more oxides of Mg, Si, Ca, Ti, Zr, C, Ni, Y, and rare earth elements. U.S. Re. 32,843 and Re. 34,446 disclose fabrication of alumina-based ceramics comprising silicon carbide whiskers using a hot-press method.
However, despite the research into developing high density alumina-based ceramics, there still remains a need for a method that enables the efficient production of alumina-based ceramics with a density greater than 97% of the theoretical density for the ceramic and which has improved thermal conductivity, strength, and toughness, and increased resistance to wear, corrosion, and oxidation.
The present invention provides a process for producing alumina matrix carbide and boride reinforced ceramic composites wherein for any particular composite, the relative density is 97% or more of the theoretical density. The composites are preferably prepared in a container wherein the interior surfaces of the container are graphite and have a protective coating consisting of a first layer comprising silicon carbide and boron carbide with a binder and a second layer comprising silicon carbide particles, wherein the protective coating prevents carbon bleed-through and provides boride. During firing, the protective coating provides boride in an equilibrium atmosphere in the container wherein the boride is at a concentration that inhibits leeching of the boron or boron carbide from the green preform. The equilibrium atmosphere also prevents carbon from outside the container from entering the container and impregnating the green preform. Thus, the equilibrium atmosphere enables alumina-based ceramics to be fabricated to higher densities. The present invention further provides an alumina-based ceramic composite which comprises a metal carbide preferably selected from the group consisting of silicon carbide, titanium carbide and zirconium carbide, and mixtures thereof, and a boride preferably selected from the group consisting of boron carbide, titanium boride, or zirconium boride, and mixtures thereof. Finally, the present invention provides a protective coating for an article comprising a first layer of a silicon carbide and a boron carbide in a binder and a second layer comprising silicon carbide particles wherein the protective coating is able to withstand repeated exposure to high temperature.
Thus, the present invention provides a process for preparation of a dense alumina-based ceramic composition which comprises: (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried; (b) introducing into the container a dried green preform made from a mixture of an alumina and a metal carbide powder and a boride powder, wherein the mixture has been milled together; (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.
In particular, the present invention provides a process for preparation of a dense alumina-based ceramic composition which comprises: (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried; (b) introducing into the container a dried green preform made from a mixture of an alumina and a metal carbide powder and a boride powder, wherein the mixture has been milled together; (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.
In a preferred embodiment, the present invention provides a process wherein the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures thereof. Optionally, the present invention further provides a process wherein composition comprising the preform green includes a sintering aid selected from the group consisting of yttria, rare earths, magnesia, calcia, lanthanides, and mixtures thereof. Preferably, the sintering aid is yttria (Y2O3).
The present invention further provides a process for preparation of a dense alumina-based ceramic composition which comprises: (a) providing a container with a removable closure, wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried; (b) introducing into the container a dried green preform made from a mixture of an aluminum oxide, a silicon carbide powder, and a boron carbide powder, wherein the mixture has been milled together; (c) firing the preform at a temperature sufficient to produce the ceramic composition which has a density of at least 97 percent of a theoretical density for the ceramic composition.
In particular embodiments of the present invention the first layer comprises 98 wt % of the silicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt % of the organic binder and the silicon carbide particles are of about 70 to 120 mesh.
In the process of the present invention the firing is performed by ramping the temperature to about 500xc2x0 C. at a rate of between about 1xc2x0 to 5xc2x0 C. per minute, then to about 1250xc2x0 C. to 1600xc2x0 C. at a rate of 1xc2x0 to 10xc2x0 C. per minute, then to a temperature of between about 1600xc2x0 to 1900xc2x0 C. at a rate of about 1xc2x0 to 20xc2x0 C. per minute, and then maintaining the preform at the temperature of 1600xc2x0 to 1900xc2x0 C. for a time sufficient to achieve a minimum density of 97%, preferably a time between about 10 to 120 minutes. Alternatively, the firing is performed by ramping the temperature to about 500xc2x0 C. at a rate of between about 1xc2x0 to 5xc2x0 C. per minute, then to about 1600xc2x0 C. at a rate of 5xc2x0 to 20xc2x0 C. per minute, and then maintaining the preform at the temperature of 1600xc2x0 to 1900xc2x0 C. for a time sufficient to achieve a minimum density of 97%, preferably a time between about 10 to 120 minutes. In a preferred embodiment of the present invention, the firing is performed in an atmosphere consisting of an inert gas wherein the gas is selected from the group consisting of argon, helium, nitrogen, and mixtures thereof.
In the process of the present invention, the green preform comprises 65 to 85 wt % of the alumina with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, 0.5 to 20 wt % of the boride with a median particle size of not more than 30 d50 xcexcm, and 2 to 21.4 wt % of the metal carbide with median particle size in the size range of 2 to 10 d50 xcexcm. Therefore, the present invention provides a preform green composition for producing a ceramic comprising 65 to 85 wt % of the alumina with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, 0.5 to 20 wt % of the boride with a median particle size of not more than 30 d50 xcexcm, and 2 to 21.4 wt % of the metal carbide with median particle size in the size range of 2 to 10 d50 xcexcm. Thus, the present invention further provides a fired ceramic composite comprising 65 to 85 wt % of the alumina with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, 0.5 to 20 wt % of the boride with a median particle size of not more than 30 d50 xcexcm, and 2 to 21.4 wt % of the metal carbide with median particle size in the size range of 2 to 10 d50 xcexcm.
In particular, the present invention provides the composition and the fired ceramic composite wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 5.4 wt % with a median particle size in the size range of 3 to 11 d50 xcexcm, and the metal carbide is a mixture of silicon carbide which is 2 wt % with a median particle size in the size range of 2 to 10 d50 xcexcm and 12.6 wt % with a median particle size in the size range of 2 to 10 d50 xcexcm; or wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 5.4 wt % with a median particle size in the size range of 3 to 11 d50 xcexcm, and the metal carbide is silicon carbide and is 14.6 wt % with a median particle size in the size range of 0.5 to 1.0 d50 xcexcm; or wherein the alumina is 78.1 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 0.5 wt % with a median particle size not more than 12 d50 xcexcm, and the metal carbide is silicon carbide and is 21.4 wt % with a median particle size in the size range of 0.5 to 1.0 d50 xcexcm; or wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d50 xcexcm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d50 xcexcm; or wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is titanium boride, e.g., TiB2, and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d50 xcexcm, and the silicon carbide is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d50 xcexcm. The carbide and/or boride particles can be a powder or whiskers. The green preform is made into a shape by slip casting, pill pressing, cold isostatic pressing, extrusion forming, injection molding, or dry bag pressing.
Further still, the present invention provides a protective coating for a surface that is exposed to high temperatures wherein the surface is graphite prepared by a process comprising providing a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer on the surface which is then coated with a second layer comprising metal carbide particles which is then dried to form the protective coating. Preferably, a protective coating for a surface that is exposed to high temperatures wherein the surface is graphite prepared by a process comprising providing a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer on the surface which is then coated with a second layer comprising metal carbide particles which is then dried to form the protective coating. In particular, the protective coating wherein the first layer comprises 98 wt % of the silicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt % of the organic binder and wherein the second layer of silicon carbide particles is 70 to 120 mesh.
The present invention further provides a container with a removable closure for firing ceramics wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of metal carbide particles, boride particles, and an organic binder in water to form a first layer which is then coated with a second layer of metal carbide particles to form a coating which is then dried. Preferably, a container with a removable closure for firing ceramics wherein inside surfaces of the container and closure are graphite and have been first coated with a mixture of silicon carbide powder, boron carbide powder, and an organic binder in water to form a first layer which is then coated with a second layer of silicon carbide particles to form a coating which is then dried. In particular, a container wherein the first layer comprises 98 wt % of the silicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt % of the organic binder and wherein the second layer of silicon carbide particles is 70 to 120 mesh.
Further still, the present invention provides an industrial blast nozzle assembly comprising: a ceramic composite liner having a bore extending therethrough to provide an inlet opening and an outlet opening wherein the ceramic composite liner which comprises 65 to 85 wt % of an alumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of a metal carbide has a density of at least 97% of a theoretical density for the ceramic; and a metal casing having a bore extending therethrough, wherein the liner is mounted in the bore of the metal casing. In a preferred embodiment, the metal casing is a metal selected from the group consisting of brass and aluminum. In a particular embodiment of the nozzle assembly, the metal casing has a threaded end and the liner is mounted in the bore of the metal casing such that the threaded end of the metal casing and the inlet end of the liner form an end which is substantially flush. Optionally, the nozzle assembly further comprises a protective coating which binds together the liner and metal casing. Preferably, the protective coating is polyurethane. In a preferred embodiment of the liner in the nozzle assembly, the inlet opening has a wider diameter than the outlet opening and there is a venturi shape in the bore between the inlet and the outlet openings. In particular embodiments of the liner for the nozzle assembly, the alumina has a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride has a median particle size of not more than 30 d50 xcexcm, and the metal carbide has a median particle size in the size range of 2 to 10 d50 xcexcm. In a preferred embodiment of the liner for the nozzle assembly, the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures thereof.
Further still, the present invention provides a liner for an industrial blast nozzle assembly comprising a ceramic composite having a bore extending therethrough to provide an inlet opening and an outlet opening wherein the ceramic composite which comprises 65 to 85 wt % of an alumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of a metal carbide has a density of at least 97% of a theoretical density for the ceramic. In a preferred embodiment of the liner, the inlet opening has a wider diameter than the outlet opening and there is a venturi shape in the bore between the inlet and the outlet openings. In particular, the present invention provides a liner wherein the alumina has a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride has a median particle size of not more than 30 d50 xcexcm, and the metal carbide has a median particle size in the size range of 2 to 10 d50 xcexcm. Preferably, the metal carbide is selected from the group consisting of silicon carbide, titanium carbide, zirconium carbide, and mixtures thereof and the boride is selected from the group consisting of boron carbide, titanium boride, zirconium boride, and mixtures thereof.
In particular embodiments of the liner for the industrial blast nozzle, the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 5.4 wt % with a median particle size in the size range of 3 to 11 d50 xcexcm, and the metal carbide is a mixture of silicon carbide which is 2 wt % with a median particle size in the size range of 2 to 10 d50 xcexcm and 12.6 wt % with a median particle size in the size range of 2 to 10 d50 xcexcm; or wherein the alumina is 80 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 5.4 wt % with a median particle size in the size range of 3 to 11 d50 xcexcm, and the metal carbide is silicon carbide and is 14.6 wt % with, a median particle size in the size range of 0.5 to 1.0 d50 xcexcm; or wherein the alumina is 78.1 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 0.5 wt % with a median particle size not more than 12 d50 xcexcm, and the metal carbide is silicon carbide and is 21.4 wt % with a median particle size in the size range of 0.5 to 1.0 d50 xcexcm; or wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is boron carbide, e.g., B4C, and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d50 xcexcm, and the metal carbide is silicon carbide and is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d50 xcexcm; or wherein the alumina is 60 to 85 wt % with a median particle size in the size range of 0.4 to 1.5 d50 xcexcm, the boride is titanium boride, e.g., TiB2, and is 1 to 20 wt % with a median particle size in the size range of 1 to 30 d50 xcexcm, and the silicon carbide is 5 to 15 wt % with a median particle size in the size range of 0.5 to 20 d50 xcexcm.
Therefore, it is an object of the present invention to provide a process for producing an alumina matrix carbide and boride reinforced ceramic composites which have a relative density greater than 97% of the theoretical density.
It is a further object of the present invention to provide a protective coating for an article which protects the article from repeated exposure to a carbon atmosphere.
Further still, it is an object of the present invention to provide an industrial blast nozzle assembly wherein the nozzle assembly has ceramic liner comprising an alumina-based composite containing carbides and borides which has a relative density greater than 97% of the theoretical density.
These and other objects of the present invention will become increasing apparent with reference to the following description of preferred embodiments and examples.